The zebrafish is an excellent vertebrate model organism for both genetic and experimental embryological studies. We plan to use the zebrafish to study genetic control of important developmental pathways, such as hematopoiesis or blood formation. The study will include chemical mutagenesis of the DNA in sperm cells followed by genetic breeding and identification of fish with alterations in blood formation. These fish will be used to identify genes involved in the development of the blood. These studies may elucidate genes and their encoded proteins essential for hematopoiesis. Further, an understanding of the development of the blood components may help us to study the mechanism of leukemia formation. We have conducted a pilot mutagenesis screen and uncovered several mutant fish lines that have defects specifically in one branch of hematopoiesis that give rise to white blood cells. Studying one of these mutants indicate that it has defects in several steps in hematopoiesis and the efforts to pinpoint the gene mutated are ongoing. We have also started a larger scale mutagenesis screening which hopefully will lead to the discovery of many more mutants. Studying such mutants will give us a much better understanding of the genetic control of hematopoiesis.